Abstract [en]

It is not well-known how the accumulated pool of atmospheric lead pollution in the boreal forest soil will affect the groundwater and surface water chemistry in the future as this lead migrates through the soil profile. This study uses stable lead isotopes (206Pb/207Pb and 208Pb/ 207Pb ratios) to trace the transport of atmospheric lead pollution within the soil of a small catchment and predict future lead level changes in a stream draining the catchment. Low 206Pb/207Pb and 208Pb/207Pb ratios for the lead in the soil water (1.16 ± 0.02; 2.43 ± 0.03) and streamwater (1.18 ± 0.03; 2.42 ± 0.03) in comparison to that of the mineral soil (>1.4; >2.5) suggest that atmospheric pollution contributes by about 90% (65−100%) to the lead pool found in these matrixes. Calculated transport rates of atmospheric lead along a soil transect indicate that the mean residence time of lead in organic and mineral soil layers is at a centennial to millennial time scale. A maximum release of the present pool of lead pollution in the soil to the stream is predicted to occur within 200−800 years. Even though the uncertainty of the prediction is large, it emphasizes the magnitude of the time lag between the accumulation of atmospheric lead pollution in soils and the subsequent response in streamwater quality.

In thesis

Klaminder, Jonatan

Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.

2005 (English)Doctoral thesis, comprehensive summary (Other academic)

Abstract [en]

Lead has a more than three-millennia-long pollution history in Europe. Metal production, burning of coal and use of leaded petrol resulted in a significant pollution of the atmosphere. As a consequence of atmospheric fallout, the Swedish boreal forest is strongly contaminated by airborne lead pollution. High levels of lead in the soil and soil pore water are of concern because the soil fauna, plants and aquatic biota may respond negatively to this toxic element. The fate of the accumulated pool of pollution in the soil is not well known.

In this thesis, I determine four variables of importance for increasing our knowledge about the past, present and future lead levels in the boreal forest soil: 1) the pre-pollution atmospheric deposition rate of lead; 2) the upward flux of lead from deeper soil layers to near-surface horizons as a result of plant uptake; 3) the mean residence time of lead in the mor layer (the organic horizon at the surface of forest soils); and 4) the vertical and lateral transport of pollution lead within the mineral soil and to streams. Lead concentration measurements and stable lead isotope analyses were used for distinguishing pollution lead from natural lead in cores from ombrotrophic bogs, forest soil profiles, forest mosses, soil-water and stream-water samples.

The results clearly stress that the boreal forest ecosystem is totally dominated by pollution lead. This is proved by low 206Pb/207Pb ratios (mainly between 1.14 and 1.20) in the mor layer, forest plants and stream water, while the local geogenic lead of the mineral soil (C-horizon) has high ratios (> 1.30). The dominance of pollution in the mor layer is caused by high deposition rates of airborne lead pollution, minute transport rates of lead from the mineral soil by forest plants (about 0.02 mg lead m-2 year-1) and a long mean residence time of the deposited lead (~250 years for mature forest). In the pristine pre-pollution environment, lead was a rare element due to low atmospheric deposition rates (0.001 to 0.01 mg m-2 year-1). It is estimated that the present lead inventory in the mor layer is up to 100 times higher than in the pristine environment where ≤ 8 mg m-2 was present in the mor. The levels in this biologically important horizon will decrease at a very slow rate and it will take centuries for the deeper part of the mor layer to fully respond to decreasing atmospheric inputs. In a hypothetic scenario with a ceased atmospheric lead deposition, the pool of pollution lead will ultimately be redistributed to deeper water-saturated soil layers from where a lateral transport to surface waters occurs. In the studied catchment, the export of pollution lead from the soil to the stream is estimated to peak slightly about one thousand years from now.